An optical lens is typically made of plastic or glass material and generally has two opposing surfaces which co-operate with one another to converge or diverge light according to a required corrective prescription. When the positioning or shape of one of these surfaces with respect to the other is inaccurate, optical errors can be created.
Manufacturing of an optical lens to the required prescription requirements typically includes machining the surface of an unfinished lens member such as a semi-finished lens or a lens blank. Typically, a semi-finished lens has a finished surface, for example the front surface and an unfinished opposing surface, for example the back surface. By machining the unfinished surface of the lens to remove material, the required shape and positioning of a back surface with respect to a front surface for the desired corrective prescription can be generated.
During manufacturing of the lens, the semi-finished lens is securely maintained by blocking the semi-finished lens on a blocker. Various techniques may be used to secure the semi-finished lens on the blocker. These include using low temperature fusible alloys, or glue or applying vacuum techniques to hold the semi-finished lens in place.
It is important to secure the semi-finished lens at an accurate positioning on the blocker during the various manufacturing operations in order to prevent the generation of optical errors. With the development of optical lens of increasing superior quality, the necessity for accurate positioning is becoming increasingly important for the correct orientation of the back surface with respect to the front surface. Errors in positioning may include operator error, machine inaccuracy,
Current techniques for ensuring that the positioning of a lens member with respect to a blocker is as accurate as possible during manufacturing of an optical lens includes verifying the positioning of the semi-finished lens during the blocking step by methods including an operator visualising the lens positioning directly or by observing the positioning of the semi-finished lens by means of a camera, and then correcting the positioning if necessary. The semi-finished lens is placed on the blocker and the target markings of a positioning camera are used to position the semi-finished lens. For this purpose a semi-finished lens is often provided with marking features including surface markings such as engraved marks, or internal markings to define a reference framework of the design of the finished surface of the lens. Factors such as mechanical shock can however influence the accuracy of the camera visualisation.
The present invention has been devised to address one or more of the foregoing concerns.